Preload unit modules of this type are used in transmissions in motor vehicles, in order in particular to permit the actuation of a friction clutch in the drive train of a motor vehicle.
German patent DE 10 2005 053 555 B3 discloses an axial displacement device in the form of a ball ramp arrangement. The axial displacement device comprises two disks centered on a common axis, one of which is supported axially and the other is axially displaceable, and of which at least one can be driven in rotation. The two disks in each case have on their mutually facing end faces an equally large multiplicity of ball grooves running in the circumferential direction. The two disks are therefore mounted with balls as rolling bodies.
U.S. Pat. No. 5,485,904 likewise discloses a preload unit of which the ramp disks are arranged such that they can be rotated with respect to each other by means of balls as rolling bodies.
Likewise, U.S. Pat. No. 5,620,072 discloses a preload unit for a multi-plate clutch, the ramp disks of which are likewise arranged such that they can be rotated with respect to each other by means of balls as rolling bodies.
FIG. 1 shows, schematically, the use of a preload unit 1 according to the prior art. A preload unit 1 is used, for example, in an all-wheel coupler unit and, as illustrated in FIG. 1, is composed substantially of a casing 60, in which a second multi-plate clutch 54, a first multi-plate clutch 52 and the preload unit 1 are arranged in accordance with the prior art. The casing 60 of the all-wheel coupler unit has a transmission input shaft 56 and a transmission output shaft 58. The preload unit 1 comprises a first ramp disk 2 and a second ramp disk 5. Between the two ramp disks 2 and 5 there are between five and six balls, which constitute the rolling bodies 3. Formed in the ramp disks 2 and 5 are in each case corresponding ramps (not illustrated in FIG. 1) or ramp contours 8 (not illustrated in FIG. 1), in which the rolling bodies 3 roll. The ramps 9, 10 are formed obliquely, which effects low-friction displacement of the two ramp disks 2, 5 in the direction of the axis 50. With the displacement of the two ramp disks 2 and 5, a lift in the direction of the axis 50 can thus be achieved. One of the ramp disks 2 or 5 can be engaged electromagnetically by means of the first multi-plate clutch 52.
FIG. 2 shows a schematic illustration of a preload unit 1 as known from the prior art. The preload unit 1 comprises a first ramp disk 2 and a second ramp disk 5. Between 5 and 6 rolling bodies 3 are provided between the two ramp disks 2 and 5. The rolling bodies 3 of the prior art are formed as balls. The second ramp disk 5 is followed by a thrust disk 7, which is mounted with respect to the second ramp disk 5 by an axial bearing 6. As can be seen from the illustration in FIG. 2, a first pitch circle 62, on which the rolling bodies 3 are arranged, has a smaller diameter than a second pitch circle 64, on which the axial bearing 6 is arranged. As a result of the difference between the first pitch circle 62 and the second pitch circle 64, when the preload unit 1 is loaded, a force deflection occurs. This has the disadvantage that, as a result, it is possible for deformations and high mechanical stresses of the first ramp disk 2 and the second ramp disk 5 to occur. The axial force flow 66 is thus not completely parallel to the axis 50.
German patent DE 10 2004 015 271 B4 discloses a torque transfer device. Here, the rolling bodies and the axial bearing are fitted on a pitch circle which has the same diameter. Balls are used as rolling bodies.
An axial preload unit is likewise employed in the bevel gear differentials to prestress multi-plate clutches of the superimposition stages.
As described in FIG. 1 and FIG. 2, the second ramp disk 5 is provided with internal toothing and is connected to the transmission output shaft 58 and thus to the rear axle (not illustrated). Between the transmission input shaft 56 and the transmission output shaft 58 there is the second multi-plate clutch 54. When the transmission input shaft 56 and the transmission output shaft 58 rotate at different speeds (e.g. when the rear wheels are on ice), the ramp disks 2 and 5 of the preload unit 4 rotate relative to each other. The normal force resulting from this actuates the second multi-plate clutch 54. The torque can thus be transmitted from the transmission to the rear axle. In order to save fuel on the highway, the plates of the preload unit 1 can be disengaged electromagnetically. In this case, the drive is provided only to the front wheels of the motor vehicle.